Japanese Patent Laid-Open No. 10-183316 discloses a surface treatment method, in surface treatment for a casting material such as a mating surface with respect to the cylinder block of a cylinder head, of forcing, while rotating, a rotary tool having a projection formed at its distal-end shoulder, thereby stirring in a nonmelted state with heat.
A portion between port openings (valve-to-valve portion) of a cylinder head repeats volume expansion due to combustion of the engine and volume shrinkage due to cooling, and cracking caused by thermal fatigue tends to occur there. Therefore, the valve-to-valve portion is surface-treated by frictional stirring to realize a metal with a finer structure and to decrease the area ratio of the cast tunnels, so a resistance against generation of cracking caused by thermal fatigue and propagation of cracking increases, thereby improving the thermal fatigue strength.
Due to the presence of the port openings, the rigidity of the material is low in the valve-to-valve portion. The pushing force and rotation moment of the rotary tool plastically deform the matrix structure of the side surfaces of the openings in the vertical and horizontal directions, and accordingly the shapes of the port openings change undesirably.
When the shapes of the port openings change, the material which has plastically fluidized by frictional stirring moves to the port opening side. Then, a void tends to occur in the surface modification region.